Drilling fluids have a number of functions, including but not limited to, lubricating the drilling tool and drill pipe which carries the tool, providing a medium for removing formation cuttings from the well to the surface, counterbalancing formation pressure to prevent the inflow to the wellbore of gas, oil, and/or water from permeable or porous formations which may be encountered at various levels as drilling progresses, preventing the loss of drilling fluids to void spaces and to permeable or porous formations, maintaining hole stability prior to setting the casing, minimizing formation damage, and holding the drill cuttings in suspension, especially in the event of a shutdown in drilling and interruption of pumping of the drilling mud.
Drilling fluid additives in time can form a thin, low permeability filter cake (mud cake) that can seal openings in formations to reduce the unwanted influx of fluids or the loss of drilling fluids to permeable formations. The mud cake forms when the drilling fluid contains particles that are approximately the same size as or have diameters greater than about one third of the pore diameter (or the width of any openings such as induced fractures) in the formation being drilled. Drilling fluid additives can also increase the stability of the wellbore.
The drilling fluid must circulate in the wellbore (down the drill pipe and back up the annulus) in order to perform all of the desired functions to allow the drilling process to continue. Therefore, drilling fluid should remain in the wellbore all the time, otherwise well control and caving in of the wellbore is immediate. Sometimes penetration of undesirable formation conditions causes substantial to severe losses of the drilling fluid to the formation. The features responsible for such losses can be related to small to large fissures, or natural or induced fractures in the formation; the losses may also be through highly porous rock.
Fluid loss is a common occurrence in drilling operations. Drilling fluids are designed to seal porous formations intentionally while drilling; this occurs as the result of suction of the fluid onto the permeable surface (pressure greater in the well than in the formation) and the creation of a mud cake to seal a porous formation during drilling and for the purpose of wellbore stabilization. Some fluid will be lost through the mud cake and fluid loss control additives are required.
However, the loss of fluids (the whole slurry) to the formation can reach an extent such that no mud cake can be created to secure the surface and create an effective barrier. In extreme situations, when the borehole penetrates a fracture in the formation through which most of the drilling fluid may be lost, the rate of loss may exceed the rate of replacement. Drilling operations may have to be stopped until the lost circulation zone is sealed and fluid loss to the fracture is reduced to an acceptable level. In the worst case, the consequences of this problem can be loss of the well.
Curing losses effectively and quickly is still a matter of concern for many companies and operators. The volumes of mud loss and the amounts of lost circulation material used are both very great. Over the years numerous techniques have been developed in order to cure or to reduce low to moderate lost circulation of mud to the wellbore. Under these conditions, the normal procedure is to add fluid loss agents. The mechanism is to change the Theological properties of the drilling mud in order to increase the resistance to flow of the fluid to the formation. This alone may decrease the losses while drilling to an acceptable level. However, when it comes to intolerable losses, it is now traditional to add various bulk materials known as LCMs. Such prior art lost circulation materials are selected from different groups of materials in the form of flakes (or laminated), granular, and fibrous materials. Materials commonly used are cheap wastes from other industries and they are not engineered to be effective at blocking the loss zone. Not all materials are capable of building a blocking material with low permeability. The material may not only be dislodged and allow further fluid loss, but also may not pack in the required way to be able to block losses. Regarding fibers, the situation is similar; not all fibers are capable of blocking fluid loss under given conditions, and selection and use of the wrong fiber can cause great complications in mixing and pumping and ultimately have no blocking effect.
Organic and inorganic, natural and synthetic fibers have also been used along with other particles and/or polymeric treatments. For example, in a cure for lost circulation, fibers were added to aqueous solutions of partially hydrolyzed polyacrylamide (U.S. Pat. No. 5,377,760). In another example, a high fluid loss spotting pill included a carrier fluid, a LCM containing acidizable mineral particulates, preferably calcium carbonate (fine and coarse particles and fine flakes) along with acid-soluble mineral fibers, preferably fine fibers such as extruded mineral wool having a diameter of 4 to 20 microns, preferably from about 5 to 6 microns and a length of fiber about 200 microns, preferably 8 to 25 microns (US 2003/0195120 and U.S. Pat. No. 6,790,812). In yet another example, a loss circulation combination contained alkali metal silicate and water-insoluble particulate material as an integral component of a water-based drilling fluid system and a quantity of water-soluble activating agent effective to reduce the pH of the water-based drilling fluid system low enough to cause precipitation of the silicate; the water-insoluble particulate materials included cellulose fibers selected from corn cobs, nut shells, seeds, pith, and lignin and had sizes from about 0.025 to about 2 mm (about 10 to about 500 mesh) (US 2007/0034410 and U.S. Pat. No. 7,226,895). Coconut coir in the form of a mixture of short fibers, flakes, granular pieces, and powder from the coconut husk has been used as an LCM in drilling fluid to prevent loss of drilling fluid into fractures in rock formations (US 2004/0129460). Finally, glass fibers or novoloid fibers in an amount of 1.43 to 17.12 kg/m3 (0.5 to 6 lb/bbl) have been mixed with solid particles of less than 300 microns (WO 2004/101704 and US 2007/0056730).
Despite all these efforts, there is still a need for a precisely engineered material that can quickly and efficiently reduce severe lost circulation, and that can build a strong barrier in the loss zone, in wells being drilled with oil-based muds.